1. Field of the Invention
This invention relates to electronic supports measures (ESM) systems and, more particularly, to apparatus for detecting an erroneous frequency measurement made by an instantaneous frequency measurement (IFM) receiver using a multi-channel delay line discriminator.
2. Description of Related Art
An instantaneous frequency measurement of a detected radar signal is commonly made in an electronic support measures (ESM) system in order to identify the source of the detected radar signal.
In a typical system, the detected radar signal is sent through a delay line. This causes the signal to undergo a phase shift which is directly related to the frequency of the detected signal. The phase shift is then measured and, based thereon, the frequency of the detected signal is calculated.
To maximize band width and measuring accuracy, the detected radar signal is often delivered simultaneously to several delay lines of progressively greater length and each resulting phase delay is measured. Such a system is known as a multi-channel delay line discriminator. All phase shift measurements are then fed to a frequency resolver which analyses them and, based thereon, provides an output indicative of the actual frequency of the detected radar signal.
In such a system, the phase shift measurements are typically delivered in the form of a multi bit digital word commonly referred to as a "measured phase code." The output from the frequency resolver is also typically delivered in the form of a multi bit digital word. It is commonly referred to as the "measured frequency word."
A significant problem in such IFM systems is the loss of accuracy which results when two incoming signals are present at the same time or when the signal level is too low. This often causes an error in one or more of the measured phase codes and, often (but not always) as a consequence, in the measured frequency word.
Attempts have been made to avoid this problem by sensing the existence of the error-creating problem, i.e., the presence of two signals at once or a signal of insufficient level.
Unfortunately, such efforts have generally required expensive and complicated circuitry. They also have not generally worked well.